The upgrade was started from panel and finished without errors. The textual modus runs well (F1-F6). The GUI desktop runs about 1000% slower, for example, to open then bash window (terminal) needs some minutes.

1 Answer
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Unless you have been using ppas to ensure you are always running the latest version of KDE, as part of the upgrade you have probably taken a leap of several versions. Sometimes these don't go very well as the config of the old version might not suit the new one.

Go to home directory and ensure you are showing hidden files in dolphin (view/show hidden files). open .kde/share/config and rename all the files that begin with plasma.
Reboot and you should be presented with a clean desktop that will hopefully be running properly. In doing it this way you will have preserved the configuration of all your apps, but you will need to reconfigure your desktop.